Wow, controlling the gain level sounds incredibly useful, and exactly why I posed the question.
I was about to set it up as a basic volume pedal, but it occurs to me that the Axe could make much more interesting use out of it: like controlling gain. I'd love to hear from someone who has it set up like that!
There's also the whole "morphing" thing, where you use a pedal to change many elements of your tone simultaneously.
Back in the early days of my experience with midi using the ART SGX-2000 (and later with a Johnson JM150 modeling amp), I resorted to doing this because I couldn't stand the dropouts during patch changes. I had 5 or 6 parameters all tied to 1 expression pedal and I would use it to morph from a clean sound to a lead sound.
Forum member Clarky has posted a whole methodology on doing this.